Re: SSD + RAID

2009/11/13 Greg Smith <greg(at)2ndquadrant(dot)com>:
> In order for a drive to work reliably for database use such as for
> PostgreSQL, it cannot have a volatile write cache.  You either need a write
> cache with a battery backup (and a UPS doesn't count), or to turn the cache
> off.  The SSD performance figures you've been looking at are with the
> drive's write cache turned on, which means they're completely fictitious and
> exaggerated upwards for your purposes.  In the real world, that will result
> in database corruption after a crash one day.  No one on the drive
> benchmarking side of the industry seems to have picked up on this, so you
> can't use any of those figures.  I'm not even sure right now whether drives
> like Intel's will even meet their lifetime expectations if they aren't
> allowed to use their internal volatile write cache.

hm. I never understood why Peter was only able to turn up 400 iops
when others were turning up 4000+ (measured from bonnie). This would
explain it.

Is it authoritatively known that the Intel drives true random write
ops is not what they are claiming? If so, then you are right..flash
doesn't make sense, at least not without a NV cache on the device.

merlin